1. Field of the Invention
This invention relates to collision resolution in a delay-critical radio telecommunications system, especially to resolution of collision in the Random Access Channel (RACH) in the General Packet Radio Service (GPRS) and Enhanced Data rate for GSM Evolution (EDGE).
2. Description of the Related Art
Cellular mobile communication systems such as the Global System for Mobile communications (GSM) make use of RACHs in order to enable the initial access of the mobile stations to the network. Packet radio networks (like GPRS and EDGE) also make use of similar channels called Packet Random Access Channels (PRACH) not only for the initial access but also during the call since channels are allocated to users on a demand basis, rather than permanently (as in circuit switched GSM). The random access mechanism used in these systems is based on Slotted ALOHA, as described in L. G. Roberts, “ALOHA packet system, with and without slots and capture”, ACM Computer Communication Review, vol. 5, no. 2, pp. 28-42, Apr. 1975. The mobile station (MS) transmits a short message over the (P)RACH which occupies one single radio burst. Normally, the position (frequency and timeslot) of the PRACH is indicated by the Broadcast Channel (BCCH). The main problem with ALOHA techniques comes from the fact that the transmission on the PRACH is not centrally coordinated and therefore, several MSs might access the PRACH at the same time. This is referred to as collision. Collisions may have a significant impact on the overall system performance especially for real time services. As an example, the transmission of packet voice over a packet switched radio interface is considered. Since voice is an alternating sequence of active and inactive periods (talkspurts and silence gaps respectively) a channel (combination of frequency and timeslot) is normally allocated to an MS only when it needs to transmit a talkspurt. After the end of the talkspurt transmission, the channel is given to another MS. At the beginning of the talkspurt, an MS needs to access the PRACH in order to indicate its activity and to request a channel. If the base station successfully receives the access message and if a channel is currently available, it sends an acknowledgement message to the MS indicating that a channel has been allocated for its use. In a circuit switched scenario, this process only takes place at the beginning of a call while in a packet switched voice system it happens on the average every 2 sec. However, the whole access procedure is subject to several error mechanisms:    a) Physical layer errors may occur during the uplink transmission of the access message due to co-channel interference and thermal noise;    b) Several MSs access the PRACH simultaneously and therefore, a collision occurs;    c) Physical layer errors may occur during the downlink transmission of the acknowledgement message.Any of these failure mechanisms contributes to the system performance degradation. If the access phase is not successful, the MS needs to access the PRACH again while speech packets are stored in a temporary buffer. However, in case the buffer overflows, speech packets are dropped. This is known as speech front-end clipping and may significantly deteriorate the quality of oral communication.
It is known that under certain conditions it can be possible for one access packet to be successfully decoded, even if several messages (from different MS) occur simultaneously on the random access channel. This is known as capture effect. Capture can significantly improve the performance of ALOHA-type systems and methods are described by C. Namislo, “Analysis of mobile radio slotted ALOHA systems”, IEEE Journal on Selected Areas in Communications, vol. SAC-2, no. 4, pp. 583-588, July 1984 and by H. Zhou, R. H. Deng, “Capture model for mobile radio slotted ALOHA systems”, IEE Proc. Communications, vol. 145, no. 2, pp. 91-97, Apr. 1998. Most of the capture models presented in the open literature so far are based on power differences between the packets simultaneously accessing the random access channel (power capture). However, power capture is not an appropriate model for microcellular or picocellular environments where most of the MS are close to the base station and furthermore, (signal-based) power control techniques which aim at equalizing the power from different MS over the cell area are employed.